The purpose of this study is 1) to assess the effect of 4 mg oral BRL 49653C b.i.d. administered for 12 weeks vs. placebo on fasting and post-prandial hepatic (overall glucose production and gluconeogenesis) and muscle (overall glucose disposal, glycolysis, oxidation, and storage) glucose metabolism in patients with non-insulin dependent diabetes mellitus (NIDDM), and 2) to describe the safety of 4 mg oral BRL 49653C b.i.d. in patients with NIDDM.